In the field of aircraft construction, composite structural members are widely used which have a matrix of thermosetting resin acting as a binder for elongate reinforcing elements. The reinforcing elements are customarily parallel tapes of continuous filaments (or tows) of carbon fibre that are held by a line of stitching so as to form sheets which are cut to size and placed, superposed, in a moulding fixture which is then covered with a vacuum bag and introduced into an autoclave, applying temperature and pressure such as to bring about the polymerization of a thermosetting resin. In some cases sheets of parallel carbon fibres pre-impregnated with resin are used. In other cases sheets that are not pre-impregnated are used, being superposed in the dry state on the moulding fixture, and in the vacuum bag a measured amount of resin is previously arranged which, in the autoclave, permeates the sheets of fibre and polymerizes. This latter technique is known in the art as resin film infusion or resin transfer moulding in the case where use is made of a closed fixture with resin injection for polymerization in an autoclave.
Both the above-mentioned methods give unsatisfactory results when the structural member has a complex three-dimensional shape, particularly when it has curvatures in two non-parallel planes. This is because in order to adapt the sheets to the curved moulding fixture on which they are superposed it is difficult, if not impossible, to arrange the carbon fibres, which are parallel and adjacent in each sheet, according to the course (curve) of the fixture. As a result, the fibres are not oriented according to the curvilinear development of the structural member, and therefore are not oriented according to the maximum stress to which the structural member will be subjected in practice. It is further necessary to avoid the sheets of fibres forming wrinkles or areas that are excessively stretched and thinned out, or that have slits, since this produces a discontinuity or interruption of the path of the stresses that the fibres must withstand in practice.
Some techniques have been proposed (curved braiding, overbraiding) which make it possible to obtain three-dimensional solids (termed “dry preforms” in the art) constituted by fibres with curvilinear or radial orientation, but these techniques are suitable only for producing components of constant thickness. Furthermore, the formation of a dry preform on a curved mandrel is disadvantageous, since the preform must be manipulated and transferred together with the mandrel, otherwise the shape imparted may be lost.